A cable television (CATV) system distributes a broadband RF signal to individual subscriber locations. The headend originates the RF signal, and the distribution network divides, or distributes, the signal to each subscriber location. Amplifiers and line extenders carry the RF signal to a local distribution point near a group of subscriber locations. A tap is used to "tap" some of the RF signal from the local distribution point and deliver it to the subscriber's home via a coaxial drop cable. Typically, a tap is a 2, 4, or 8 port device, which respectively provides service to 2, 4, or 8 subscriber locations.
Taps vary in both the numbers of subscriber output ports and the level of RF signal (the tap value) that is tapped from the input RF signal. Tap values are expressed in dB of attenuation from the input RF signal. Most taps divert only a portion of the input RF signal to subscriber output ports and pass almost all of the input signal level to the output connector. A directional coupler is not linear in tap value to insertion loss correlation (insertion loss is the signal attenuation from input to output) the higher the tap value the less insertion loss in the directional coupler. A 20 dB directional coupler will only have about 0.8 dB of insertion loss where as a 9 dB directional coupler will have about 2.5 dB of insertion loss Some taps are terminating taps that have subscriber output ports but no output connector.
A typical tap has a directional coupler and an internal splitter network. The directional coupler taps a fixed level of RF signal from the input connector, and passes the tapped RF signal through the internal splitter network. The subscriber output ports are connected to the splitter outputs. The output of the directional coupler is connected to the tap output connector, which couples input RF signal less the directional couplers insertion loss to the next tap in the CATV system. A terminating tap (which has no output connector) includes only a splitter network and subscriber output ports for connection to local subscriber drop lines.
In very dense subscriber locations, it is known to cascade several 2, 4 or 8 port taps together in one location to serve 24, 32 or even 48 homes. In such case, each tap is selected for a desired value and required number of ports. The same tap housing accommodates any one of a variety of possible tap variations. That is, a typical tap housing is designed to hold any one of a number of different tap configurations, and a given line of taps will fit the common tap housing design. To increase the number of subscriber ports at a given location, additional tap housings are added in series.
However, in order to add additional tap housings in series, the downstream side of the CATV system is disconnected, which interrupts service to the downstream subscribers. In addition, as more subscribers are added, amplification and/or equalization of the RF signal may become necessary to avoid loss of quality of the RF signal. Installing an amplifier or equalizer will interrupt service to downstream subscribers. As CATV services upgrade to include advanced services such as telephone or Internet access, even brief interruptions in service is unacceptable.
In areas with a large number of subscribers, it is also known to use large multi-port, multi-value tap configurations that incorporate fixed cascades of 4 and 8 port taps with different tap values in a single housing. Multiple large tap product versions include 16, 32, and 48 port models with bandwidths ranging form 5 MHz to 1000 MHz. Different tap values in one large tap housing permit the use of different lengths of subscriber drop lines while still providing proper signal levels to the television sets.
The advantage of a large multi-port, multi-value tap housing as compared to cascading separate 8-port taps, is that large tap configurations are less expensive, are specifically designed for the application (which enhances the RF performance) and require less space. However, the number of possible large tap variations leads to a large number of product models. Also, during field service, if one of the internal taps or subscriber ports is found to be damaged or defective, the whole multi-port, multi-value large tap may have be replaced.